X-rays are conventionally used to examine and evaluate the interior of dense materials and are also used in the medical evaluation of humans. In order to record the images produced by these evaluations, it has been conventional to employ X-ray intensifying screens containing a suitable phosphor to convert the X-ray energy to a more useful UV-visible light. The light emitted by the phosphor will then expose a conventional silver halide element in contact with the screen and thus produce the desired record.
The X-ray screens are conventionally fabricated by using a suitable phosphor mixed in a slurry with a binder and coated on some sort of conventional support such as cardboard or polyester film, for example. The useful phosphors are usually prepared by mixing the starting materials together and firing the mixture at elevated temperatures in various atmospheres, e.g., nitrogen, hydrogen, etc. The phosphor is then washed to remove unreacted starting materials and slurried with a suitable binder as previously described. After coating, a protective topcoat or abrasion coat may be applied thereover in order to extend the usable life of the finished screen.
While there are many known materials that can luminesce under the influence of impinging X-ray energy, only a special few have those properties that are requisite for use as an X-ray intensifying screen. These include the well-known calcium tungstate phosphors, as well as the M' monoclinic tantalates described by Brixner in U.S. Pat. No. 4,225,653. These tantalate phosphors are extremely efficient in converting X-ray energy to UV light and are now widely used. Also to be mentioned are the lanthanum and gadolinium oxyhalides similar to those described by Brines and Rabatin in U.S. Pat. No. 4,499,159.
Images produced by silver halide elements used with the aforementioned X-ray intensifying screen elements, must be sharp and clear especially when they are used to record X-ray evaluations of the human body. Speed is also an important factor since it is deleterious to expose the human body to overdoses of X-rays. Thus, there is a special need to maintain the high speed of X-ray intensifying screen elements to minimize radiation exposure while at the same time insuring that a quality image, free of noise, is produced.
The use of mixed phosphors is also known in the prior art For example, Patten in U.S. Pat. No. 4,387,141 describes a mixed phosphor comprising calcium tungstate and the tantalates of the aforementioned Brixner patent to achieve improved speed and sharpness and low noise.
Since tabular grain silver halide elements are becoming more popular there is also a need to match the characteristics of these light-sensitive grains with the light output of various phosphors and still achieve good image quality. The tantalate phosphors described above along with phosphors such as gadolinium oxysulfide, are useful with these silver halide elements. However, there is a continuing requirement to improve the speed and sharpness of these tabular grain/phosphor systems.
It is an object of this invention to provide a mixed phosphor X-ray intensifying screen that will produce high speed and higher resolution than that of the individual phosphor components when exposed to X-rays using either tabular or conventional, e.g., spherical or semi-spheroidal, grain silver halide elements.